A phase tracking multichannel link (PMTL) is a link for preserving the relative phases of multiple input signals. A PTML accepts multichannel input signals and outputs signals that replicate the input signals except for the introduction of a signal time delay for delay line applications or a signal space displacement for link applications, with mutual input phases otherwise being preserved. A PTML is therefore intended to preserve the existing phase relationship between the input signals, across time (μsec) or over distance (km), regardless of changes in temperature or other environmental parameters.
As an example of its utility, the input signals to the PTML may be provided from a phased array antenna that samples the phases from a received radio frequency plane wave. An appropriate coherent receiver determines the wavefront direction from this collection of relative phases. Until now, this receiver has had to be located near the antenna to minimize the phase tracking errors generated by the connection between the antenna and receiver. This co-location was necessary because remote transmission between the antenna and receivers incurred unacceptable phase tracking errors. Currently, the connection method would be to use parallel coaxial RF cables or optical fiber cables with a different cable connecting each antenna element and the corresponding receiver channel. Phase tracking errors would occur in this link when the coaxial cables' physical properties are not precisely matched and are subjected to a change in temperature or other physical environments. Similarly, separate parallel fibers may not be precisely matched and therefore have different refractive or expansive indices, causing a change in phase with temperature or other physical environment. When the environmental changes differ for each fiber or cable, the propagation path length and consequent phase will change differentially by the refractive properties or by the expansion properties of even precisely matched cables or fibers. Each cable would require matched properties and matched lengths, and containment within a homogeneous environment over lengths of hundreds of meters to achieve phase tracking. Phase matching separate transmission lines over this distance is therefore extremely difficult in practice.
A practical example illustrates the magnitude of the difficulty of phase tracking. If two broadband (3 to 5-Gigahertz) signals were to be transmitted over separate 612-meter fiber links, a 3-microsecond delay in optical fiber, and if 5 degrees of phase tracking were to be maintained, the time delay over the optical link would be required to match to within 2.8 picoseconds, corresponding to 0.5 millimeter. If multiple lengths of identical fiber were matched this precisely, to 1 part-per-million (ppm), a subsequent non-uniform variation in the environment or non-uniform material response to the environment could readily eliminate the match. Even a 1-degree Celsius non-uniformity between fibers would cause an unacceptable 7-ppm (35 degree) mismatch, for example.
Therefore, the common approach is to place the receiver directly behind the antenna in an attempt to avoid these problems by minimizing the link length. This is the reason for co-locating a shipborne phased array antenna with the associated receiver. PTML allows a more desirable configuration with only the antenna mounted on a mast and the receiver located below the decks. This would reduce size and weight problems on the mast, eliminate environmental problems for the receiver, and eliminate the design constraints on the receiver caused by locating it on the mast.
PTML is also required in other applications. One such application is extracting multichannel signals from a hostile environment where interchannel phase must be preserved. Such hostile environments as temperature, moisture, or electromagnetic interference (EMI), for example, would not damage the antenna, which is passive, but would damage a receiver with sensitive electronic components. Another example for an application is a delay line for phase coherent signals.